Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses and at different energies.
FIG. 1 depicts a traditional ion implanter system 100 in which a technique for low-temperature ion implantation may be implemented in accordance with an embodiment of the present disclosure. As is typical for most ion implanter systems, the system 100 is housed in a high-vacuum environment. The ion implanter system 100 may comprise an ion source 102, biased to a potential by power supply 101, and a complex series of beam-line components through which an ion beam 10 passes. The series of beam-line components may include, for example, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet collimator 110, and a second deceleration (D2) stage 112. Much like a series of optical lenses that manipulate a light beam, the beam-line components can filter and focus the ion beam 10 before steering it towards a target wafer. During ion implantation, the target wafer is typically mounted on a platen 114 that can be moved in one or more dimensions (e.g., translate, rotate, and tilt) by an apparatus, sometimes referred to as a “roplat.”
With continued miniaturization of semiconductor devices, there has been an increased demand for ultra-shallow junctions. For example, tremendous effort has been devoted to creating better activated, shallower, and more abrupt source-drain extension junctions to meet the needs of modern complementary metal-oxide-semiconductor (CMOS) devices.
To create an abrupt, ultra-shallow junction in a crystalline silicon wafer, for example, an amorphization of the wafer surface may be desirable. Generally, a relatively thick amorphous silicon layer may be preferred since a thin amorphous layer may allow more significant channeling, and so a deeper as-implanted dopant atoms depth distribution and more post-implant damage residing in an end-of-range area beyond the amorphous-crystalline interface. As a result, a thinner amorphous layer may lead to a deeper junction depth, a less abrupt doping profile, an inadequate activation of dopants, and more end-of-range defects after anneal, all of which represent major obstacles in modern CMOS device miniaturization, especially for source-drain extension doping. Amorphization of a silicon wafer can be achieved with a pre-amorphization implant (PAI) process. So far, silicon, germanium, or inert gas atomic ions and some exotic molecular ion species have been used in PAI processes.
To further ensure the formation of a shallow yet abrupt junction, a low-thermal-budget anneal is often performed as a preferred post-implant process wherein the temperature of a wafer is ramped up to a high level in a very short time (e.g., to 1000° C. within 5 seconds). A laser or a flash lamp may also be employed for the post-implant anneal. However, the diffusion-less anneal alone may not be enough to prevent all the ion-implanted dopants from diffusing deeper into the wafer. A process known as transient enhanced diffusion (TED), which is driven by excess silicon interstitials created during dopant implantation, can cause a significant amount of certain dopants (e.g., boron, phosphorous) to diffuse further into the wafer. It is possible for the diffusion coefficient of the ion-implanted dopants to increase temporarily by orders of magnitude until the implant damage has been annealed out. It has been discovered that certain species such as carbon (C) and fluorine (F) may reduce the TED effect by reducing the interaction between interstitials and dopant atoms. One existing approach employs a cluster implantation process to place carbon into silicon wafers in order to reduce the TED effect. However, this approach requires not only proprietary cluster implantation equipment, but also exotic, proprietary hydrocarbon molecules as feed materials. Another approach uses atomic species as co-implant materials.
In view of the foregoing, it would be desirable to provide techniques for forming shallow junctions which overcomes the above-described inadequacies and shortcomings.